A Study in Advanced or Metastatic Gastrointestinal Cancers Exploring Treatment Combinations With Pelareorep and Atezolizumab

Cohorts 1-5 Inclusion Criteria: * ECOG performance status of 0 or 1 * Have measurable lesions per RECIST v1.1 * Patients must have adequate hematological, renal, and hepatic function * Have recovered to ≤grade 1 or baseline for all adverse events (AEs) due to previous therapies or surgeries. * For female patients of childbearing potential and male patients with partners of childbearing potential, agreement to use a highly-effective form(s) of contraception and to continue its use for 6 months after the last dose of study drug. Exclusion Criteria: * Undergone systemic chemotherapy, radiotherapy, or surgery, \<4 weeks before study treatment. * Received previous treatment with immune checkpoint inhibitors * Uncontrolled or severe cardiac disease * Active, uncontrolled infections * Symptomatic brain metastasis * Interstitial lung disease with symptoms or signs of activity. * Autoimmune disease that has required systemic treatment in the past 2 years with disease modifying agents, corticosteroids, or immunosuppressive drugs. * A seizure disorder that requires pharmacotherapy. * Known hypersensitivity to any of the study drugs, study drug classes, or excipients in the formulation. * A non-healing wound, non-healing ulcer, or non-healing bone fracture within 4 weeks prior to the start of study drug. * Women who are pregnant or breastfeeding. * A diagnosis of immunodeficiency or is receiving chronic systemic steroid therapy or any other form of immunosuppressive therapy * Any vaccine within 28 days prior to first treatment or during the first cycle of study treatment. Exclusion Criteria: * In Cohort 1, 2, 3, 4: Life expectancy less than 3 months * In Cohort 1, 2, 3: known active Hepatitis B (HBV) or Hepatitis C (HCV) infection that requires anti-viral treatment. * In Cohort 4: Prior HIV infection if the CD4+ T cell is \<300 cells/µl * In Cohort 5: Known low or absent dihydropyrimidine dehydrogenase (DPD) activity. * In Cohort 5: Known leptomeningeal disease. * In Cohort 5: History of another primary cancer within the last 3 years with the exception of non-melanoma skin cancer, early-stage prostate cancer, or curatively treated cervical carcinoma in-situ